The present invention relates to a device and a method for detecting the position and velocity of a test object relative to a sensor, the sensor and the test object being arranged such that they can be displaced relative to one another, wherein the sensor has a measuring coil with at least one, or preferably a number of voltage tap(s) and has a target, which is placed on the test object while being electromagnetically coupled to the measuring coil and wherein an electronic component for adding the voltages tapped on the sensor is assigned to the sensor.
Measuring devices of the type mentioned above are required, for example, for determining the position of the piston on pneumatic and hydraulic cylinders and also for measuring valve positions, particularly in closed loop control circuits. It is advantageous for such applications if the position of the test object and its velocity of movement relative to the sensor can be measured simultaneously and absolutely.
Measuring devices for determining the position of a valve have been known for a long time. Thus, for example, the printed literature DE 198 56 528 A1 describes a valve lift sensor for determining the position of outlet valves and inlet valves, said sensor comprising two stator bodies spaced apart, which have circular recesses for a sensor plunger. A Hall sensor is arranged between the stator bodies and at a distance therefrom. The cylindrically designed sensor plunger is provided with a permanent magnet, which is displaced relative to the Hall sensor. The small dimension of the Hall sensor proves to be advantageous for this embodiment. However, only the position of the valve can be determined using this sensor.
In other displacement measuring sensors known from the prior art (for example, from the printed literature DE 25 11 683 C3 or DE 39 13 861 A1), a secondary winding is wound on an elongated magnetically soft sensor core and two primary windings are wound on the ends of the sensor core. A magnet displaced along the sensor core serves as the test object. By impinging current upon the primary winding, a voltage is induced in the secondary winding depending on the position of the magnet, which applies the magnetically soft sensor core in saturation to its immediate vicinity. The disadvantage here is the large dimensioning of the sensor. In addition, only the position can be determined here.
Furthermore, an eddy current sensor is known from DE 42 25 968 A1, in which a measuring coil is provided with a number of voltage taps. By means of a ring, which surrounds the measuring coil and is made of an electrically conductive material, the partial impedance of the measuring coil between two adjacent voltage taps is influenced depending on the position of the ring. With the help of an evaluation circuit, it is thus possible to determine the relative position of the ring in relation to the voltage taps. However, the only direct result of this method is the position.
A solenoid valve, particularly for inlet and outlet valves of internal combustion engines is described in the printed literature DE 197 35 375 C1. For regulating the application of current to the magnetic drive mechanism, the position of the valve armature is detected by means of a piezoelectric element measuring the force of the valve spring. From this it is possible to determine the respective position of the armature and thus the stroke course and the speed of the armature can be determined. What proves to be disadvantageous here is the strongly restricted structure of the sensor and the determination of the speed from the course of the armature position over time.
The crucial disadvantage of the methods known from practical experience for determining the velocity from the displacement signal by differentiating the signal is that disturbances particularly noise, greatly influence the output signal in differentiating systems in general. Thus there are a strongly increasing number of requirements made of the quality of the displacement signal, particularly in relation to linearity and the signal-to-noise ratio, thereby necessitating the use of expensive low-noise sensors.
In principle, two sensors can also be used, of which one detects the course of the position over time while the other sensor detects the velocity of the test object. However, in doing so, the space required and the costs of the sensor system increase considerably due to the sensors and the evaluation electronic systems required in each case. Furthermore, an additionally necessary synchronization of the two sensors has negative effects.
It is therefore an object of the present invention to specify a device and a method of the type mentioned in the introduction for detecting the position and velocity of a test object, in which method it is possible to measure both the position and the velocity using only one sensor simultaneously and without subsequent differentiation of the displacement signal as cost-effectively as possible, with long term stability and with the smallest possible dimensions of the device.